问题补充：
I often doubt .
A. that this song is worth listening to
B. if this song is worth listening
C. that this song is worth listening
D. if this song is worth listening to
答案：
A. that this song is worth listening to
(答案→)B. if this song is worth listening
C. that this song is worth listening
D. if this song is worth listening to
解析：本题的含义是我经常怀疑这首歌是否值得去听，if引导一般疑问句的宾语从句，表示是否的意思，be worth doing sth表示值得做某事，后面没有名词或代词，listen无需加to，故本题选B。
